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Square wave voltammetric detection of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene on a gold electrode modified with self-assembled monolayers
Sensors and Actuators B 133 (2008) 509–515
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Square wave voltammetric detection of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene on a gold electrode modified with self-assembled monolayers Robert G. Bozic a,∗ , Alan C. West a,1 , Rastislav Levicky b,2 a b
Department of Chemical Engineering, Columbia University, New York, NY 10027, United States Department of Chemical & Biological Engineering, Polytechnic University, Brooklyn, NY 11201, United States
a r t i c l e
i n f o
Article history: Received 21 November 2007 Received in revised form 10 January 2008 Accepted 12 March 2008 Available online 25 March 2008 Keywords: 2,4,6-Trinitrotoluene 2,4-Dinitrotoluene Square wave voltammetry Alkanethiol self-assembled monolayers
a b s t r a c t A rotating disc gold electrode, either bare or modified with an alkanethiol self-assembled monolayer, was used to measure the simultaneous square wave voltammetric differential current response of the electrochemical reduction of the two ordnance-related compounds (ORCs) 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT). On bare electrodes, background currents limited sensitivity to ppm levels. In order to suppress these interfering currents, the gold electrodes were modified with alkanethiols of various chain lengths, from 3 to 12 carbons, with hydrophilic (hydroxyl) or hydrophobic (methyl) end groups presented to the electrolyte. Gold electrodes endowed with hydroxyl-terminated monolayers enabled clear detection of TNT and DNT from mixtures of these two compounds. In addition, longer alkanethiols allowed extension of the working potential range to more negative values than achieved with shorter alkanethiols. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In 1976, the Resource Conservation and Recovery Act (RCRA) initiated monitoring of areas in which there is disposal of solid hazardous waste. Based on RCRA, ordnance-related compounds (ORCs) are considered to be hazardous solid waste; therefore, their presence at parts per billion concentration levels (g/L) in ORC disposal areas must be monitored for possible remediation [1–4]. The cost of monitoring ground water for ORCs based on the Environmental Protection Agency’s regulation for the disposal of solid waste, SW-846, prompted an active search for alternatives. A recent cost projection of the US Army’s long term monitoring program was nearly $500 million over 10 years. The Department of Energy projected $100 million per year over 70 years and the Navy estimated $80 million per year [5]. The present testing method requires site sample collection and laboratory analysis using high-performance liquid chromatography [6]. Up to 70% of the program cost arises from the need to perform sample collection and analysis at different locations [5].
∗ Corresponding author at: Department of Chemical Engineering, Columbia University, 500 West 12th Street, Room 801 Mudd, New York, NY 10027, United States. Tel.: +1 212 854 4546. E-mail addresses: [email protected] (R.G. Bozic), [email protected] (A.C. West), [email protected] (R. Levicky). 1 Tel.: +1 212 854 4452. 2 Tel.: +1 718 260 3682. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.03.017
The search for an alternative and more cost-efficient method of analysis has spurred interest in on-site electrochemical detection of ordnance-related compounds as well as other approaches, including spectrophotometric and immunoassay methods [7–9]. Over two decades ago, Bratin et al. investigated the electrochemical reduction of nitroaromatics, nitramines, and nitrate esters coupled with liquid chromatography [10]. Reduction potentials and redox chemistry of the nitro groups associated with 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) were reported. Electrochemical methods have also been used for analysis of ORCs in soil samples [11]. More recent developments include an electrochemical sensor for TNT using a carbon fiber electrode [4,12]. While this was proven to be an effective method for TNT detection, it does not demonstrate the ability to sense TNT and DNT simultaneously as commonly found in a mixture of contaminated ground water. These previous findings establish electrochemical detection as an attractive option for on-site monitoring of ORCs in ground water because the required hardware is portable and robust. Electrochemical sensing can also be integrated within a microfluidic processing station so that required sample handling and detection are all performed within a single, self-contained device. Such an integrated platform for ORC detection would provide a fast, inexpensive analytical alternative to curtail transportation costs associated with current procedures. Square wave voltammetry (SWV) has been used in ORC detection studies of ppm concentrations of TNT on a carbon working electrode [2–4]. SWV is a current sampling technique that is par-
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ticularly sensitive because it diminishes some of the effects of background currents. In the context of the present work, use of gold electrodes has the further advantage in developing integrated microfluidic systems because of the ubiquity of gold deposition technologies. Moreover, the facile and precise modification of gold electrodes with alkanethiol self-assembled monolayers (SAMs) provides a versatile approach to tuning their electroanalytic performance. Alkanethiol SAMs are a well-established method of modifying gold surfaces [13,14] and their utility for ORC detection is explored in the present communication. The SWV results presented in this investigation of the simultaneous electrochemical reduction of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene were collected using a gold electrode with and without a SAM. Alkanethiols of different chain lengths and different end terminus, hydroxyl versus methyl, were tested in SWV assays aiming to quantify ORC concentrations in solutions containing TNT, DNT, and mixtures of these compounds. Other ORC signatures of interest include, for example, trinitro benzene (TNB) and nitro benzene (NB) [15]. In general, it is of interest to identify which ORCs, from various species of potential interest, are suitable for multiplex detection in a mixture. Here it is shown that SAMs endowed the electrode surface with different capacities for hydrophobic and hydrogen bonding interactions with the analyte, in some cases facilitating and in others suppressing the ability to detect TNT and DNT ORCs relative to a bare Au electrode. 2. Experimental 2.1. Chemicals TNT and DNT ampules, 1000 g/mL in acetonitrile, were obtained from Cerilliant of Round Rock, Texas. Samples for analysis were prepared by dilution of the as-obtained solutions in 0.5 M sodium chloride or 0.1 M phosphate buffer (PB), pH 7. PB was prepared from appropriate combination of monobasic and dibasic sodium phosphate (Fisher). Because the ORC analyte was provided as a solution in acetonitrile, it was desirable to fix the acetonitrile concentration during measurement, irrespective of the dilution factor used. Accordingly, acetonitrile (HPLC grade, Honeywell) was added in order to maintain a concentration of 1:100 (v/v) acetonitrile:water throughout the experiments. As needed, hydrochloric acid and sodium hydroxide (Fisher) were used for pH adjustments. All alkanethiols were obtained from Sigma–Aldrich, and included methyl-terminated 1-hexanethiol, 1-decanethiol, and 1-dodecanethiol, in addition to hydroxylterminated 3-mercapto-1-propanol (MCP), 6-mercapto-1-hexanol (MCH), and 11-mercapto-1-undecanol (MCU). All chemicals were used as received. 2.2. Apparatus and procedure Square wave voltammetric measurements were carried out on a Autolab III/FRA 2 potentiostat from Brinkman using an E4 series change tip gold disk insert (5.0-mm o.d. × 4.0-mm thick, polished) working electrode. The experiments used a silver/silver chloride reference electrode with a 3-M sodium chloride reservoir (Ag/AgCl/3 M NaCl RE; BASI), and a platinum wire counter electrode. All potentials are reported relative to the Ag/AgCl/3 M NaCl reference. The rotator was from Pine Instruments of Raleigh, North Carolina. Experiments were carried out at a rotation speed of 500 rpm. Prior to measurement, the working electrode was mechanically polished with 600 and p4000 grit silicon carbide paper from Leco Corps and Buehler, followed by polishing with 1 m ultra fine dia-
mond polish on a one micron ultra polishing pad from BASI. After mechanical polishing, the electrode was rinsed with methanol and then de-ionized water. The electrode was next placed in a 0.1-M sulfuric acid, 0.01 M potassium chloride bath and the potential was cycled for 20 scans from 0 to 1.54 V using a 100-mV/s scan rate. This procedure produces a reproducible initial state of the electrode surface [16]. Following this electrochemical polishing, the electrode was again rinsed with de-ionized water. For measurements at a bare Au electrode, the prepared working electrode was directly placed into ORC-free PB or NaCl electrolyte for conditioning (see below). Alternately, the working electrode was derivatized with an alkanethiol monolayer. SAM modification was carried out by exposing the electrode to a solution containing 0.1 mM alkanethiol. MCP and MCH were dissolved in de-ionized water, while ethanol was used for other alkanethiols. The reaction time allowed for formation of the SAMs varied. For MCP and MCH the reaction time was 40 min. For the longer chain MCU and 1-dodecanethiol, the electrode was left in the alkanethiol solution for 10 h. The working electrode (either bare or with a SAM) was first conditioned in solution absent of ORCs using five square wave voltammetric scans at 15 Hz, 4.05 mV step height, and 25 mV amplitude. For bare Au electrodes, the potential range was from 0 to −1.2 V. For electrodes modified with an alkanethiol layer, the potential range was decreased from 0 to −0.7 V for MCP, from 0 to −0.8 V for MCH, and from 0 to −0.9 V for MCU and dodecanethiol. Following this conditioning, a final SWV scan was performed to serve as a background trace. Next, an aliquot of ORC solution was added. After 1 min of mixing, a square wave voltammetric scan was carried out. This procedure was then repeated over a series of stepwise additions of ORC solution, increasing the concentration of TNT or DNT each time. Error bars, when shown, are 1 S.D. from the mean value.
3. Results and discussion Previous studies of the reduction of TNT and DNT indicate the electron deficient nitrogen in the nitro groups acts as an electron acceptor [4,10]. The electron transfer, studied in an acidic solution, indicated that the complete reduction of the nitro groups occurred in two steps. The first step is from the nitro to hydroxylamine groups, and the second from the hydroxylamine to amine groups as illustrated in Fig. 1 [4,10]. More recent work with a carbon fiber working electrode supported the current understanding of the reduction reactions for TNT [4]. The primary goal of the present contribution was to optimize resolution and detection of TNT and DNT analytes, which are commonly found together, from mixture samples at a gold working electrode. The selection of a gold working electrode was motivated by the fact that gold features are highly amenable to microfabrication needed for eventual manufacture of self-contained, integrated microfluidic packages to allow on-site sample analysis. The simplest approach would be to characterize the composition of TNT and DNT mixture samples at a bare gold electrode directly, without any chemical modification of the electrode surface. To this end, a set of initial experiments was carried out on a rotating disc electrode using square wave voltammetry to monitor the reduction signatures of DNT and TNT. The SWV technique is particularly attractive for diagnostic applications because it affords suppression of non-analyte (background) current contributions [17]. Two different supporting electrolytes were employed, either 0.1 M phosphate buffer or 0.5 M NaCl, at pH 7. Throughout, the solvent was maintained as a 1:100 (v/v) acetonitrile:water solution. Fig. 2 plots raw SWV data obtained at 4 ppm analyte concentration for pure TNT, pure DNT, and for a mixture of both ORCs. In ORC-free PB, the background is dominated by a broad peak invari-
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Fig. 1. Ordnance-related compounds, reduction potentials with respect to a silver/silver chloride electrode, and reduction reaction mechanisms.
ably observed at ∼−0.2 V. This peak is believed to be associated with dissolved oxygen because it can be eliminated by sparging with nitrogen gas for 1 h. Sequential addition of DNT progressively suppresses and shifts this peak to a small feature observed at −0.3 V. For clarity only the final curve, for a DNT concentration of 4 ppm, is shown in Fig. 2. Concurrently, a pronounced DNT peak grows in at −0.61 V. This peak is expected, and reflects reduction of DNT as illustrated in Fig. 1. In comparison, TNT should produce a reduction signal near −0.5 V, the more positive potential reflecting the greater ease of reducing the trinitro compound [10]. In the TNT trace of Fig. 2, also at 4 ppm, this signal is hidden under the broad oxygen feature. Similar to behavior in DNT solutions, the feature shifts negatively but is not as effectively suppressed. In a mixture of TNT and DNT, each at 4 ppm, the beneficial effect of DNT enables resolution of both the TNT and DNT signals, with the TNT peak appearing at −0.48 V and the DNT peak at −0.62 V. The above results reveal that detection of ORCs at the bare gold electrode is complicated by other processes that generate currents comparable to those of the analyte in the 1–10 ppm range, making ORC detection at concentrations below these levels problematic. Switching the electrolyte to 0.5 M NaCl produced
Fig. 2. Representative square wave voltammograms, without background subtraction, for 4 ppm TNT analyte only, 4 ppm DNT analyte only, and for a 4 ppm DNT + 4 ppm TNT mixture. Solutions were made with 0.1 M phosphate in 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was bare gold.
somewhat different, but also complex, behavior. The complexity of the electrochemical response precluded quantitative correlation of the measured signal with ORC concentration at ppm and lower levels. A possible solution to the background problem is to deliberately modify the working electrode in order to suppress currents not related to ORC electroactivity. This was attempted by modification of the working electrode with alkanethiol films. Alkanethiols spontaneously chemisorb to gold and other noble metal surfaces via their thiol moiety to create nanometer thin layers, the so-called self-assembled monolayers. The structure and chemical composition of SAMs can be controllably adjusted to tune the surface properties [13]. In this study, several alkanethiols were tested to investigate the impact of SAM thickness and chemical functionality on the diagnostic performance. SAM thickness ranged from less than a nanometer (e.g. mercaptopropanol) to close to 2 nm (mercaptoundecanol). The terminus of the alkanethiol in contact with electrolyte was either a hydroxy (–OH) or a methyl (–CH3 ) group. Fig. 3 plots examples of SWV traces for TNT, DNT, and TNT + DNT mixture measured on a gold electrode modified with the thinnest
Fig. 3. Representative square wave voltammograms, without background subtraction, for 4 ppm TNT analyte only, 4 ppm DNT analyte only, and for a 4-ppm DNT + 4 ppm TNT mixture. Solutions were made with 0.1 M phosphate in 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was gold modified with an MCP monolayer.
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SAM considered, namely that assembled from MCP with an approximate thickness of 0.7 nm. This thickness was estimated from the mass density of MCP, d = 1.07 g/cm3 , molar mass of MCP, m = 92 g/mol, and surface coverage of alkanethiol monolayers on gold of g = 4.7 × 1014 molecules/cm2 , where t = gm/(dNA ) [18]. Compared to the bare electrode results in Fig. 2, the beneficial impact of the SAM is clearly observed in the suppression of the interfering background currents in the region 0 to −0.5 V. The measured reduction potentials for TNT and DNT were again near to expected values, falling close to −0.51 and −0.64 V, respectively. Results obtained in 0.5 M NaCl and the 0.1-M phosphate buffer electrolytes were similar. Increasing the thickness of the SAM from MCP (0.7 nm) to MCH (1.1 nm) to MCU (1.7 nm), while maintaining the terminal –OH group, did not alter the observed reduction potentials. For the MCH modification the observed potentials were −0.51 V for TNT and −0.65 V for DNT, while for an MCU electrode the potentials for TNT and DNT were −0.50 and −0.66 V, respectively. Also, there was no evidence of a systematic dependence of peak current on SAM thickness. That reduction potentials and currents were insensitive to SAM thickness is not surprising as, compared to the timescale of the measurements, electron transfer rates are known to remain fast across alkanethiol films in this range of thickness [19,20]. Film thickness did, however, influence the range of experimentally accessible potentials. This is illustrated in Fig. 4, where square wave voltammograms are compared for electrodes that are bare and modified with MCP, MCH, MCU, and 1-dodecanethiol monolayers. All data were obtained under 0.1 M phosphate buffer in the absence of ORC analyte. For MCP monolayers, surface passivation becomes ineffective at potentials exceeding −0.55 V, where an upturn in current and approach to the curve for the bare electrode (Fig. 4) suggest significant loss of the alkanethiol. Alkanethiols are known to undergo reductive desorption from gold surfaces under negative biases [21]. Also in agreement with prior reports, electrodes passivated with SAMs of longer alkanethiols remained stable to more negative potentials [21]. From among the different SAMs tested, dodecanethiol was most robust, with MCU a close second, both allowing the analytic range to be extended to below −0.8 V (Fig. 4). Fig. 5 compares SWV traces measured for a mixture of 1 ppm TNT and 1 ppm DNT on MCU and dodecanethiol electrodes. Interestingly, there was a very pronounced difference in the ORC signal when the terminal group was changed from hydroxyl to methyl. A
Fig. 4. Square wave voltammograms, in ORC-free electrolyte, for working electrodes modified with MCP, MCH, MCU, and 1-dodecanethiol, and for a bare gold electrode. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7.
Fig. 5. Background-subtracted square wave voltammograms of 1 ppm TNT + 1 ppm DNT mixtures at gold electrodes modified with MCU and with 1-dodecanethiol. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7.
clear response was observed from both ORCs at the hydrophilic, hydroxyl-rich MCU interface. In contrast, for the dodecanethiol electrode there was only a gradual, featureless current increase starting around −0.4 V, precluding a clear identification of analyte (Fig. 5). Similar featureless traces were also observed at the methyl-terminated surfaces of hexanethiol and decanethiol monolayers. These results suggest that hydrophobic and/or hydrogen bonding interactions between the reaction species (ORC analyte, hydronium ions, water; Fig. 1) and the surface of the SAM affect the reduction reaction. That a hydrogen-bonding capable material, such as the surface of the MCU SAM, would improve affinity for TNT and DNT is consistent with the known importance of hydrogen bonding to complexation of the ORC nitro groups in designing sorption materials [22]. Moreover, at methyl-terminated SAMs the ORC reduction may be suppressed relative to that at hydroxylterminated SAMs because one or more of the participating species have difficulty in approaching and reacting at the hydrophobic surface. Although at this stage the reasons for the different electroactivity of ORCs at methyl versus hydroxyl surfaces remain speculative, it is evident that MCU is the SAM of choice for TNT and DNT detection. This system was used to explore detection limits. The analyte concentration was decreased and the experiments were run again in phosphate buffer at pH 7. The resultant SWV voltammograms, measured on solutions of the pure compounds and on a TNT + DNT mixture, are shown in Fig. 6. The measured data show that contributions from both TNT and DNT are readily resolved down to 0.12 ppm (120 ppb). Because these results are still an order of magnitude away from the EPA high level contamination concentrations of 6.9 ppb for TNT and 5.7 ppb for DNT, the capabilities of measuring TNT and DNT were tested more systematically in the range 0–120 ppb. For each of the single analyte experiments, the sensitivity, limit of detection (LOD), and limit of quantitation (LOQ) were estimated by performing 10 identical experiments for each analyte concentration. The LOD is defined to be three times the standard deviation of the peak current at the lowest detectable concentration divided by the slope of the of the peak current versus concentration regression line, and the LOQ is defined as 10 times the standard deviation divided by the same slope [23]. In each case, it was found that by discarding the first scan, the reproducibility of the measurement was greatly improved, suggesting that electrode pre-conditioning should be part of the sensing methodology. Subsequent scans, after the initial
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Fig. 6. Representative square wave voltammograms for a TNT only (A), DNT only (B), and TNT, DNT mixture (C) under optimal conditions. The electrolyte was 0.1 M phosphate buffer, l:100 (v/v) acetonitrile:water solution, pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
discarded scan, were used to calculate the average peak current and standard deviation. In analyzing the data, a source of drift in the sensor was related to an increasing baseline current. By developing a correlation based on the difference between peak height Ip and baseline current Ib , measured at −0.1 V, the LOD and LOQ could be improved. Fig. 7a illustrates the variation of signal current, Is = Ip − Ib with potential for TNT electrolytes. The sensitivity for TNT and DNT (not shown) was found to be 2 A/ppm and 3 A/ppm, respectively. Fig. 7b shows nine identical scans for a solution containing 36 ppb of TNT. Based on such measurements, the LOD and LOQ were estimated. The TNT LOD and LOQ were 3 and 8 ppb, respectively, and the DNT LOD and LOQ were 10 and 33 ppb. This is close to the range of previous work by Wang and Thongngamdee using a carbon fiber working electrode [2]. These LOD and LOQ values are near the high contamination EPA levels listed above. The EPA low contamination levels are about two orders of magnitude less, at 0.11 ppb for TNT and 0.02 ppb for DNT. Reliable quantitation of ORCs across the entire range of interest, spanning from 0.01 to 10 ppb, will therefore require a pre-concentration stage. We have started work on a pre-concentration stage as part of a microfluidic study to meet this stricter requirement. Ideally, the analyte reduction signatures would be well separated to enable direct determination of the concentration of each ORC from its peak current, or from the integrated peak area. This simplest scenario, however, is not realized because the electroactive regions for TNT and DNT overlap. This overlap manifests in the fact that the apparent peak currents for TNT and DNT from a mixture, at a given concentration, exceeded those from solutions with just one analyte at the same concentration, as shown
Fig. 8. Peak current as a function of concentration for TNT analyte only and for TNT in a mixture with DNT. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7. The working electrode was gold modified with an MCU monolayer.
in Fig. 8 for TNT and observed similarly for DNT. This difference was attributed to cross-contribution of current from one ORC to the other due to overlapping regions of electroactivity. One straightforward approach to deconvolute these contributions would be to employ a least squares fit of the SWV trace for a mixture as a linear superposition of the traces from the pure compounds. Alternately, an even simpler approach is to develop a relationship between the
Fig. 7. Representative square wave voltammograms for TNT under optimal conditions. (A) Illustrates the average signal current for concentrations from 0 to 120 ppb. Fig. 7B shows nine scans that were used to calculate the average signal current at 36 ppb. The electrolyte was 0.1 M phosphate buffer, 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
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response from pure and mixed solutions of TNT and DNT. Ongoing efforts focus on integration of thus modified gold electrodes, together with development of pre-concentration stages to extend the diagnostic limits to lower analyte levels, into microfluidic platforms designed to automate and simplify on-site sample handling, when diagnostic measurements must be performed away from the capabilities of a fully equipped laboratory. Acknowledgements This work was supported by the U.S. Army Engineer Research and Development Center, ERDC, in Vicksburg. Mississippi contract number W912HZ-06-C-0034. The authors appreciate the assistance of Dr. Denise K. MacMillan of the Environmental Laboratory at ERDC.
Fig. 9. Measured concentration, as determined by application of Eqs. (1a) and (1b), as a function of the actual concentration for mixtures of DNT and TNT and for solutions containing only a single analyte. The electrolyte was 0.1 M phosphate buffer, 1:100 (v/v) acetonitrile:water pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
concentration of analyte, TNT or DNT, and the relative height of the two primary peaks observed in SWV mixture experiments at about −0.5 and −0.64 V. In this manner it was found that the experimental data can be interpreted by equations of the form CTNT (ppm) = ˛1 Ip,TNT − ˇ1 Ip,DNT
(1a)
CDNT (ppm) = ˛2 Ip,DNT − ˇ2 Ip,TNT
(1b)
where CJ is the concentration of ORC species J, Ip,J is the peak current assigned to ORC species J as measured at −0.5 V (TNT) or at −0.64 V (DNT) from mixture data, and ˛i and ˇi are fit parameters. The values of ˛1 (for TNT) and ˛2 (for DNT) were obtained, respectively, by fitting Eqs. (1a) and (1b) to data from solutions of one ORC only. For both fits, data in the range of 0.1–1.0 ppm were used. With ˛1 and ˛2 thus fixed to represent the conversion between current and concentration for the ORC of interest, the cross-current contribution from the second, interfering ORC in a mixture could be accounted for through a concentration-independent value of ˇ1 (for DNT cross-contribution to TNT) or ˇ2 (for TNT cross-contribution to DNT). Application of Eqs. (1a) and (1b) to SWV measurements on equal-concentration mixtures of TNT and DNT is illustrated in Fig. 9. With the above correction mixture and single analyte results closely superimpose; thus, it can be seen that the above approach is effective in accounting for the ORC signal crosstalk, at least within the concentration range investigated. 4. Conclusions The results from this study indicate that electrochemical detection of TNT and DNT ORCs is possible in the sub-100-ppb range using a gold electrode that is modified with an alkanethiol selfassembled monolayer. Modification of the gold electrode surface with alkanethiols improves the resolution of the reduction peaks from both ORCs and passivates against complicating background currents observed at a bare, unmodified electrode. In addition, in a comparison of hydroxyl and methyl-terminated monolayers, it was found that the end terminus presented by the monolayer to the solution can strongly affect the diagnostic signal, with a methyl-terminated monolayer of dodecanethiol nearly eliminating all diagnostic response whereas clear, strong signal was measured at hydroxyl-terminated surfaces of comparable thickness SAMs. In optimizing the detection, mercaptoundecanol provided the best combination of monolayer stability and clarity of the diagnostic
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Biographies Robert G. Bozic is a graduate student in the Department of Chemical Engineering Columbia University. He learned a Master’s of engineering degree from the University of Florida in Gainesville, FL in 1999. He is an active duty lieutenant colonel in the U.S. Army Corps of Engineers. His PhD research is on the electrochemical detection of ordnance-related compounds.
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Alan C. West is a professor of chemical engineering at Columbia University. He received his PhD in chemical engineering at the University of California, Berkeley in 1989. His research is focused on engineering applications of electrochemistry, including microfabrication processes, sensors, fuel cells, and batteries. Rastislav Levicky is Donald F. Othmer assistant professor of chemical and biological engineering at Polytechnic University in Brooklyn, New York. Dr. Levicky obtained his doctoral training in Chemical Engineering at the University of Minnesota-Twin Cities in 1996. His current research combines biointerfacial engineering, polymer science, and electrochemistry, especially as applied to fundamental mechanisms and technology of biomolecular diagnostics and sensors, and to the design of biomaterials.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Square wave voltammetric detection of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene on a gold electrode modified with self-assembled monolayers Robert G. Bozic a,∗ , Alan C. West a,1 , Rastislav Levicky b,2 a b
Department of Chemical Engineering, Columbia University, New York, NY 10027, United States Department of Chemical & Biological Engineering, Polytechnic University, Brooklyn, NY 11201, United States
a r t i c l e
i n f o
Article history: Received 21 November 2007 Received in revised form 10 January 2008 Accepted 12 March 2008 Available online 25 March 2008 Keywords: 2,4,6-Trinitrotoluene 2,4-Dinitrotoluene Square wave voltammetry Alkanethiol self-assembled monolayers
a b s t r a c t A rotating disc gold electrode, either bare or modified with an alkanethiol self-assembled monolayer, was used to measure the simultaneous square wave voltammetric differential current response of the electrochemical reduction of the two ordnance-related compounds (ORCs) 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT). On bare electrodes, background currents limited sensitivity to ppm levels. In order to suppress these interfering currents, the gold electrodes were modified with alkanethiols of various chain lengths, from 3 to 12 carbons, with hydrophilic (hydroxyl) or hydrophobic (methyl) end groups presented to the electrolyte. Gold electrodes endowed with hydroxyl-terminated monolayers enabled clear detection of TNT and DNT from mixtures of these two compounds. In addition, longer alkanethiols allowed extension of the working potential range to more negative values than achieved with shorter alkanethiols. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In 1976, the Resource Conservation and Recovery Act (RCRA) initiated monitoring of areas in which there is disposal of solid hazardous waste. Based on RCRA, ordnance-related compounds (ORCs) are considered to be hazardous solid waste; therefore, their presence at parts per billion concentration levels (g/L) in ORC disposal areas must be monitored for possible remediation [1–4]. The cost of monitoring ground water for ORCs based on the Environmental Protection Agency’s regulation for the disposal of solid waste, SW-846, prompted an active search for alternatives. A recent cost projection of the US Army’s long term monitoring program was nearly $500 million over 10 years. The Department of Energy projected $100 million per year over 70 years and the Navy estimated $80 million per year [5]. The present testing method requires site sample collection and laboratory analysis using high-performance liquid chromatography [6]. Up to 70% of the program cost arises from the need to perform sample collection and analysis at different locations [5].
∗ Corresponding author at: Department of Chemical Engineering, Columbia University, 500 West 12th Street, Room 801 Mudd, New York, NY 10027, United States. Tel.: +1 212 854 4546. E-mail addresses: [email protected] (R.G. Bozic), [email protected] (A.C. West), [email protected] (R. Levicky). 1 Tel.: +1 212 854 4452. 2 Tel.: +1 718 260 3682. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.03.017
The search for an alternative and more cost-efficient method of analysis has spurred interest in on-site electrochemical detection of ordnance-related compounds as well as other approaches, including spectrophotometric and immunoassay methods [7–9]. Over two decades ago, Bratin et al. investigated the electrochemical reduction of nitroaromatics, nitramines, and nitrate esters coupled with liquid chromatography [10]. Reduction potentials and redox chemistry of the nitro groups associated with 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) were reported. Electrochemical methods have also been used for analysis of ORCs in soil samples [11]. More recent developments include an electrochemical sensor for TNT using a carbon fiber electrode [4,12]. While this was proven to be an effective method for TNT detection, it does not demonstrate the ability to sense TNT and DNT simultaneously as commonly found in a mixture of contaminated ground water. These previous findings establish electrochemical detection as an attractive option for on-site monitoring of ORCs in ground water because the required hardware is portable and robust. Electrochemical sensing can also be integrated within a microfluidic processing station so that required sample handling and detection are all performed within a single, self-contained device. Such an integrated platform for ORC detection would provide a fast, inexpensive analytical alternative to curtail transportation costs associated with current procedures. Square wave voltammetry (SWV) has been used in ORC detection studies of ppm concentrations of TNT on a carbon working electrode [2–4]. SWV is a current sampling technique that is par-
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ticularly sensitive because it diminishes some of the effects of background currents. In the context of the present work, use of gold electrodes has the further advantage in developing integrated microfluidic systems because of the ubiquity of gold deposition technologies. Moreover, the facile and precise modification of gold electrodes with alkanethiol self-assembled monolayers (SAMs) provides a versatile approach to tuning their electroanalytic performance. Alkanethiol SAMs are a well-established method of modifying gold surfaces [13,14] and their utility for ORC detection is explored in the present communication. The SWV results presented in this investigation of the simultaneous electrochemical reduction of 2,4,6-trinitrotoluene and 2,4-dinitrotoluene were collected using a gold electrode with and without a SAM. Alkanethiols of different chain lengths and different end terminus, hydroxyl versus methyl, were tested in SWV assays aiming to quantify ORC concentrations in solutions containing TNT, DNT, and mixtures of these compounds. Other ORC signatures of interest include, for example, trinitro benzene (TNB) and nitro benzene (NB) [15]. In general, it is of interest to identify which ORCs, from various species of potential interest, are suitable for multiplex detection in a mixture. Here it is shown that SAMs endowed the electrode surface with different capacities for hydrophobic and hydrogen bonding interactions with the analyte, in some cases facilitating and in others suppressing the ability to detect TNT and DNT ORCs relative to a bare Au electrode. 2. Experimental 2.1. Chemicals TNT and DNT ampules, 1000 g/mL in acetonitrile, were obtained from Cerilliant of Round Rock, Texas. Samples for analysis were prepared by dilution of the as-obtained solutions in 0.5 M sodium chloride or 0.1 M phosphate buffer (PB), pH 7. PB was prepared from appropriate combination of monobasic and dibasic sodium phosphate (Fisher). Because the ORC analyte was provided as a solution in acetonitrile, it was desirable to fix the acetonitrile concentration during measurement, irrespective of the dilution factor used. Accordingly, acetonitrile (HPLC grade, Honeywell) was added in order to maintain a concentration of 1:100 (v/v) acetonitrile:water throughout the experiments. As needed, hydrochloric acid and sodium hydroxide (Fisher) were used for pH adjustments. All alkanethiols were obtained from Sigma–Aldrich, and included methyl-terminated 1-hexanethiol, 1-decanethiol, and 1-dodecanethiol, in addition to hydroxylterminated 3-mercapto-1-propanol (MCP), 6-mercapto-1-hexanol (MCH), and 11-mercapto-1-undecanol (MCU). All chemicals were used as received. 2.2. Apparatus and procedure Square wave voltammetric measurements were carried out on a Autolab III/FRA 2 potentiostat from Brinkman using an E4 series change tip gold disk insert (5.0-mm o.d. × 4.0-mm thick, polished) working electrode. The experiments used a silver/silver chloride reference electrode with a 3-M sodium chloride reservoir (Ag/AgCl/3 M NaCl RE; BASI), and a platinum wire counter electrode. All potentials are reported relative to the Ag/AgCl/3 M NaCl reference. The rotator was from Pine Instruments of Raleigh, North Carolina. Experiments were carried out at a rotation speed of 500 rpm. Prior to measurement, the working electrode was mechanically polished with 600 and p4000 grit silicon carbide paper from Leco Corps and Buehler, followed by polishing with 1 m ultra fine dia-
mond polish on a one micron ultra polishing pad from BASI. After mechanical polishing, the electrode was rinsed with methanol and then de-ionized water. The electrode was next placed in a 0.1-M sulfuric acid, 0.01 M potassium chloride bath and the potential was cycled for 20 scans from 0 to 1.54 V using a 100-mV/s scan rate. This procedure produces a reproducible initial state of the electrode surface [16]. Following this electrochemical polishing, the electrode was again rinsed with de-ionized water. For measurements at a bare Au electrode, the prepared working electrode was directly placed into ORC-free PB or NaCl electrolyte for conditioning (see below). Alternately, the working electrode was derivatized with an alkanethiol monolayer. SAM modification was carried out by exposing the electrode to a solution containing 0.1 mM alkanethiol. MCP and MCH were dissolved in de-ionized water, while ethanol was used for other alkanethiols. The reaction time allowed for formation of the SAMs varied. For MCP and MCH the reaction time was 40 min. For the longer chain MCU and 1-dodecanethiol, the electrode was left in the alkanethiol solution for 10 h. The working electrode (either bare or with a SAM) was first conditioned in solution absent of ORCs using five square wave voltammetric scans at 15 Hz, 4.05 mV step height, and 25 mV amplitude. For bare Au electrodes, the potential range was from 0 to −1.2 V. For electrodes modified with an alkanethiol layer, the potential range was decreased from 0 to −0.7 V for MCP, from 0 to −0.8 V for MCH, and from 0 to −0.9 V for MCU and dodecanethiol. Following this conditioning, a final SWV scan was performed to serve as a background trace. Next, an aliquot of ORC solution was added. After 1 min of mixing, a square wave voltammetric scan was carried out. This procedure was then repeated over a series of stepwise additions of ORC solution, increasing the concentration of TNT or DNT each time. Error bars, when shown, are 1 S.D. from the mean value.
3. Results and discussion Previous studies of the reduction of TNT and DNT indicate the electron deficient nitrogen in the nitro groups acts as an electron acceptor [4,10]. The electron transfer, studied in an acidic solution, indicated that the complete reduction of the nitro groups occurred in two steps. The first step is from the nitro to hydroxylamine groups, and the second from the hydroxylamine to amine groups as illustrated in Fig. 1 [4,10]. More recent work with a carbon fiber working electrode supported the current understanding of the reduction reactions for TNT [4]. The primary goal of the present contribution was to optimize resolution and detection of TNT and DNT analytes, which are commonly found together, from mixture samples at a gold working electrode. The selection of a gold working electrode was motivated by the fact that gold features are highly amenable to microfabrication needed for eventual manufacture of self-contained, integrated microfluidic packages to allow on-site sample analysis. The simplest approach would be to characterize the composition of TNT and DNT mixture samples at a bare gold electrode directly, without any chemical modification of the electrode surface. To this end, a set of initial experiments was carried out on a rotating disc electrode using square wave voltammetry to monitor the reduction signatures of DNT and TNT. The SWV technique is particularly attractive for diagnostic applications because it affords suppression of non-analyte (background) current contributions [17]. Two different supporting electrolytes were employed, either 0.1 M phosphate buffer or 0.5 M NaCl, at pH 7. Throughout, the solvent was maintained as a 1:100 (v/v) acetonitrile:water solution. Fig. 2 plots raw SWV data obtained at 4 ppm analyte concentration for pure TNT, pure DNT, and for a mixture of both ORCs. In ORC-free PB, the background is dominated by a broad peak invari-
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Fig. 1. Ordnance-related compounds, reduction potentials with respect to a silver/silver chloride electrode, and reduction reaction mechanisms.
ably observed at ∼−0.2 V. This peak is believed to be associated with dissolved oxygen because it can be eliminated by sparging with nitrogen gas for 1 h. Sequential addition of DNT progressively suppresses and shifts this peak to a small feature observed at −0.3 V. For clarity only the final curve, for a DNT concentration of 4 ppm, is shown in Fig. 2. Concurrently, a pronounced DNT peak grows in at −0.61 V. This peak is expected, and reflects reduction of DNT as illustrated in Fig. 1. In comparison, TNT should produce a reduction signal near −0.5 V, the more positive potential reflecting the greater ease of reducing the trinitro compound [10]. In the TNT trace of Fig. 2, also at 4 ppm, this signal is hidden under the broad oxygen feature. Similar to behavior in DNT solutions, the feature shifts negatively but is not as effectively suppressed. In a mixture of TNT and DNT, each at 4 ppm, the beneficial effect of DNT enables resolution of both the TNT and DNT signals, with the TNT peak appearing at −0.48 V and the DNT peak at −0.62 V. The above results reveal that detection of ORCs at the bare gold electrode is complicated by other processes that generate currents comparable to those of the analyte in the 1–10 ppm range, making ORC detection at concentrations below these levels problematic. Switching the electrolyte to 0.5 M NaCl produced
Fig. 2. Representative square wave voltammograms, without background subtraction, for 4 ppm TNT analyte only, 4 ppm DNT analyte only, and for a 4 ppm DNT + 4 ppm TNT mixture. Solutions were made with 0.1 M phosphate in 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was bare gold.
somewhat different, but also complex, behavior. The complexity of the electrochemical response precluded quantitative correlation of the measured signal with ORC concentration at ppm and lower levels. A possible solution to the background problem is to deliberately modify the working electrode in order to suppress currents not related to ORC electroactivity. This was attempted by modification of the working electrode with alkanethiol films. Alkanethiols spontaneously chemisorb to gold and other noble metal surfaces via their thiol moiety to create nanometer thin layers, the so-called self-assembled monolayers. The structure and chemical composition of SAMs can be controllably adjusted to tune the surface properties [13]. In this study, several alkanethiols were tested to investigate the impact of SAM thickness and chemical functionality on the diagnostic performance. SAM thickness ranged from less than a nanometer (e.g. mercaptopropanol) to close to 2 nm (mercaptoundecanol). The terminus of the alkanethiol in contact with electrolyte was either a hydroxy (–OH) or a methyl (–CH3 ) group. Fig. 3 plots examples of SWV traces for TNT, DNT, and TNT + DNT mixture measured on a gold electrode modified with the thinnest
Fig. 3. Representative square wave voltammograms, without background subtraction, for 4 ppm TNT analyte only, 4 ppm DNT analyte only, and for a 4-ppm DNT + 4 ppm TNT mixture. Solutions were made with 0.1 M phosphate in 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was gold modified with an MCP monolayer.
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SAM considered, namely that assembled from MCP with an approximate thickness of 0.7 nm. This thickness was estimated from the mass density of MCP, d = 1.07 g/cm3 , molar mass of MCP, m = 92 g/mol, and surface coverage of alkanethiol monolayers on gold of g = 4.7 × 1014 molecules/cm2 , where t = gm/(dNA ) [18]. Compared to the bare electrode results in Fig. 2, the beneficial impact of the SAM is clearly observed in the suppression of the interfering background currents in the region 0 to −0.5 V. The measured reduction potentials for TNT and DNT were again near to expected values, falling close to −0.51 and −0.64 V, respectively. Results obtained in 0.5 M NaCl and the 0.1-M phosphate buffer electrolytes were similar. Increasing the thickness of the SAM from MCP (0.7 nm) to MCH (1.1 nm) to MCU (1.7 nm), while maintaining the terminal –OH group, did not alter the observed reduction potentials. For the MCH modification the observed potentials were −0.51 V for TNT and −0.65 V for DNT, while for an MCU electrode the potentials for TNT and DNT were −0.50 and −0.66 V, respectively. Also, there was no evidence of a systematic dependence of peak current on SAM thickness. That reduction potentials and currents were insensitive to SAM thickness is not surprising as, compared to the timescale of the measurements, electron transfer rates are known to remain fast across alkanethiol films in this range of thickness [19,20]. Film thickness did, however, influence the range of experimentally accessible potentials. This is illustrated in Fig. 4, where square wave voltammograms are compared for electrodes that are bare and modified with MCP, MCH, MCU, and 1-dodecanethiol monolayers. All data were obtained under 0.1 M phosphate buffer in the absence of ORC analyte. For MCP monolayers, surface passivation becomes ineffective at potentials exceeding −0.55 V, where an upturn in current and approach to the curve for the bare electrode (Fig. 4) suggest significant loss of the alkanethiol. Alkanethiols are known to undergo reductive desorption from gold surfaces under negative biases [21]. Also in agreement with prior reports, electrodes passivated with SAMs of longer alkanethiols remained stable to more negative potentials [21]. From among the different SAMs tested, dodecanethiol was most robust, with MCU a close second, both allowing the analytic range to be extended to below −0.8 V (Fig. 4). Fig. 5 compares SWV traces measured for a mixture of 1 ppm TNT and 1 ppm DNT on MCU and dodecanethiol electrodes. Interestingly, there was a very pronounced difference in the ORC signal when the terminal group was changed from hydroxyl to methyl. A
Fig. 4. Square wave voltammograms, in ORC-free electrolyte, for working electrodes modified with MCP, MCH, MCU, and 1-dodecanethiol, and for a bare gold electrode. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7.
Fig. 5. Background-subtracted square wave voltammograms of 1 ppm TNT + 1 ppm DNT mixtures at gold electrodes modified with MCU and with 1-dodecanethiol. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7.
clear response was observed from both ORCs at the hydrophilic, hydroxyl-rich MCU interface. In contrast, for the dodecanethiol electrode there was only a gradual, featureless current increase starting around −0.4 V, precluding a clear identification of analyte (Fig. 5). Similar featureless traces were also observed at the methyl-terminated surfaces of hexanethiol and decanethiol monolayers. These results suggest that hydrophobic and/or hydrogen bonding interactions between the reaction species (ORC analyte, hydronium ions, water; Fig. 1) and the surface of the SAM affect the reduction reaction. That a hydrogen-bonding capable material, such as the surface of the MCU SAM, would improve affinity for TNT and DNT is consistent with the known importance of hydrogen bonding to complexation of the ORC nitro groups in designing sorption materials [22]. Moreover, at methyl-terminated SAMs the ORC reduction may be suppressed relative to that at hydroxylterminated SAMs because one or more of the participating species have difficulty in approaching and reacting at the hydrophobic surface. Although at this stage the reasons for the different electroactivity of ORCs at methyl versus hydroxyl surfaces remain speculative, it is evident that MCU is the SAM of choice for TNT and DNT detection. This system was used to explore detection limits. The analyte concentration was decreased and the experiments were run again in phosphate buffer at pH 7. The resultant SWV voltammograms, measured on solutions of the pure compounds and on a TNT + DNT mixture, are shown in Fig. 6. The measured data show that contributions from both TNT and DNT are readily resolved down to 0.12 ppm (120 ppb). Because these results are still an order of magnitude away from the EPA high level contamination concentrations of 6.9 ppb for TNT and 5.7 ppb for DNT, the capabilities of measuring TNT and DNT were tested more systematically in the range 0–120 ppb. For each of the single analyte experiments, the sensitivity, limit of detection (LOD), and limit of quantitation (LOQ) were estimated by performing 10 identical experiments for each analyte concentration. The LOD is defined to be three times the standard deviation of the peak current at the lowest detectable concentration divided by the slope of the of the peak current versus concentration regression line, and the LOQ is defined as 10 times the standard deviation divided by the same slope [23]. In each case, it was found that by discarding the first scan, the reproducibility of the measurement was greatly improved, suggesting that electrode pre-conditioning should be part of the sensing methodology. Subsequent scans, after the initial
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Fig. 6. Representative square wave voltammograms for a TNT only (A), DNT only (B), and TNT, DNT mixture (C) under optimal conditions. The electrolyte was 0.1 M phosphate buffer, l:100 (v/v) acetonitrile:water solution, pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
discarded scan, were used to calculate the average peak current and standard deviation. In analyzing the data, a source of drift in the sensor was related to an increasing baseline current. By developing a correlation based on the difference between peak height Ip and baseline current Ib , measured at −0.1 V, the LOD and LOQ could be improved. Fig. 7a illustrates the variation of signal current, Is = Ip − Ib with potential for TNT electrolytes. The sensitivity for TNT and DNT (not shown) was found to be 2 A/ppm and 3 A/ppm, respectively. Fig. 7b shows nine identical scans for a solution containing 36 ppb of TNT. Based on such measurements, the LOD and LOQ were estimated. The TNT LOD and LOQ were 3 and 8 ppb, respectively, and the DNT LOD and LOQ were 10 and 33 ppb. This is close to the range of previous work by Wang and Thongngamdee using a carbon fiber working electrode [2]. These LOD and LOQ values are near the high contamination EPA levels listed above. The EPA low contamination levels are about two orders of magnitude less, at 0.11 ppb for TNT and 0.02 ppb for DNT. Reliable quantitation of ORCs across the entire range of interest, spanning from 0.01 to 10 ppb, will therefore require a pre-concentration stage. We have started work on a pre-concentration stage as part of a microfluidic study to meet this stricter requirement. Ideally, the analyte reduction signatures would be well separated to enable direct determination of the concentration of each ORC from its peak current, or from the integrated peak area. This simplest scenario, however, is not realized because the electroactive regions for TNT and DNT overlap. This overlap manifests in the fact that the apparent peak currents for TNT and DNT from a mixture, at a given concentration, exceeded those from solutions with just one analyte at the same concentration, as shown
Fig. 8. Peak current as a function of concentration for TNT analyte only and for TNT in a mixture with DNT. The electrolyte was 0.1 M phosphate in 1:100 (v/v) acetonitrile:water solution, pH 7. The working electrode was gold modified with an MCU monolayer.
in Fig. 8 for TNT and observed similarly for DNT. This difference was attributed to cross-contribution of current from one ORC to the other due to overlapping regions of electroactivity. One straightforward approach to deconvolute these contributions would be to employ a least squares fit of the SWV trace for a mixture as a linear superposition of the traces from the pure compounds. Alternately, an even simpler approach is to develop a relationship between the
Fig. 7. Representative square wave voltammograms for TNT under optimal conditions. (A) Illustrates the average signal current for concentrations from 0 to 120 ppb. Fig. 7B shows nine scans that were used to calculate the average signal current at 36 ppb. The electrolyte was 0.1 M phosphate buffer, 1:100 (v/v) acetonitrile:water, pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
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response from pure and mixed solutions of TNT and DNT. Ongoing efforts focus on integration of thus modified gold electrodes, together with development of pre-concentration stages to extend the diagnostic limits to lower analyte levels, into microfluidic platforms designed to automate and simplify on-site sample handling, when diagnostic measurements must be performed away from the capabilities of a fully equipped laboratory. Acknowledgements This work was supported by the U.S. Army Engineer Research and Development Center, ERDC, in Vicksburg. Mississippi contract number W912HZ-06-C-0034. The authors appreciate the assistance of Dr. Denise K. MacMillan of the Environmental Laboratory at ERDC.
Fig. 9. Measured concentration, as determined by application of Eqs. (1a) and (1b), as a function of the actual concentration for mixtures of DNT and TNT and for solutions containing only a single analyte. The electrolyte was 0.1 M phosphate buffer, 1:100 (v/v) acetonitrile:water pH 7. The working electrode was gold modified with an MCU monolayer and the temperature was 25 ◦ C.
concentration of analyte, TNT or DNT, and the relative height of the two primary peaks observed in SWV mixture experiments at about −0.5 and −0.64 V. In this manner it was found that the experimental data can be interpreted by equations of the form CTNT (ppm) = ˛1 Ip,TNT − ˇ1 Ip,DNT
(1a)
CDNT (ppm) = ˛2 Ip,DNT − ˇ2 Ip,TNT
(1b)
where CJ is the concentration of ORC species J, Ip,J is the peak current assigned to ORC species J as measured at −0.5 V (TNT) or at −0.64 V (DNT) from mixture data, and ˛i and ˇi are fit parameters. The values of ˛1 (for TNT) and ˛2 (for DNT) were obtained, respectively, by fitting Eqs. (1a) and (1b) to data from solutions of one ORC only. For both fits, data in the range of 0.1–1.0 ppm were used. With ˛1 and ˛2 thus fixed to represent the conversion between current and concentration for the ORC of interest, the cross-current contribution from the second, interfering ORC in a mixture could be accounted for through a concentration-independent value of ˇ1 (for DNT cross-contribution to TNT) or ˇ2 (for TNT cross-contribution to DNT). Application of Eqs. (1a) and (1b) to SWV measurements on equal-concentration mixtures of TNT and DNT is illustrated in Fig. 9. With the above correction mixture and single analyte results closely superimpose; thus, it can be seen that the above approach is effective in accounting for the ORC signal crosstalk, at least within the concentration range investigated. 4. Conclusions The results from this study indicate that electrochemical detection of TNT and DNT ORCs is possible in the sub-100-ppb range using a gold electrode that is modified with an alkanethiol selfassembled monolayer. Modification of the gold electrode surface with alkanethiols improves the resolution of the reduction peaks from both ORCs and passivates against complicating background currents observed at a bare, unmodified electrode. In addition, in a comparison of hydroxyl and methyl-terminated monolayers, it was found that the end terminus presented by the monolayer to the solution can strongly affect the diagnostic signal, with a methyl-terminated monolayer of dodecanethiol nearly eliminating all diagnostic response whereas clear, strong signal was measured at hydroxyl-terminated surfaces of comparable thickness SAMs. In optimizing the detection, mercaptoundecanol provided the best combination of monolayer stability and clarity of the diagnostic
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Biographies Robert G. Bozic is a graduate student in the Department of Chemical Engineering Columbia University. He learned a Master’s of engineering degree from the University of Florida in Gainesville, FL in 1999. He is an active duty lieutenant colonel in the U.S. Army Corps of Engineers. His PhD research is on the electrochemical detection of ordnance-related compounds.
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Alan C. West is a professor of chemical engineering at Columbia University. He received his PhD in chemical engineering at the University of California, Berkeley in 1989. His research is focused on engineering applications of electrochemistry, including microfabrication processes, sensors, fuel cells, and batteries. Rastislav Levicky is Donald F. Othmer assistant professor of chemical and biological engineering at Polytechnic University in Brooklyn, New York. Dr. Levicky obtained his doctoral training in Chemical Engineering at the University of Minnesota-Twin Cities in 1996. His current research combines biointerfacial engineering, polymer science, and electrochemistry, especially as applied to fundamental mechanisms and technology of biomolecular diagnostics and sensors, and to the design of biomaterials.